The present invention relates to semiconductor laser diodes, particularly to AlGaAs-based laser diodes of high light output power. Such laser diodes are commonly used in opto-electronics, often as so-called pump lasers for fiber amplifiers in the field of optical communication, e.g. for Erbium-doped fiber amplifiers. Specifically ridge-waveguide laser diodes are suited to provide the desired narrow-bandwidth optical radiation with a stable light output power in a given frequency band. Light output power and stability of such laser diodes are of crucial interest and any degradation during normal use must be avoided. The present invention concerns an improved design of such laser diodes, the improvement in particular significantly minimizing or avoiding front facet degradation of such laser diodes at very high light output powers compared to prior art designs.
Semiconductor laser diodes of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such lasers can be used for amplifying optical signals immediately by optical means. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted, which improves speed as well as reliability within such systems.
In one kind of optical fiber communication systems, the laser diodes are used for pumping Erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art. An example of some technical significance are ridge-waveguide laser diodes with a power output of 150 mW or more, whose wavelengths match the Erbium absorption lines and thus achieve a low-noise amplification. Several laser diodes have been found to serve this purpose well and are used today in significant numbers. However, the invention is in no way limited to such laser diodes, but applicable to any ridge-waveguide laser diode.
Generally, laser diode pump sources used in fiber amplifier applications are working in single transverse mode for efficient coupling into single-mode fibers and are mostly multiple longitudinal mode lasers, i.e. Fabry-Perot (or FP) lasers. Three main types are typically being used for Erbium amplifiers, corresponding to the absorption wavelengths of Erbium: InGaAsP at 1480 nm; strained quantum-well InGaAs/AlGaAs laser diodes at 980 nm; and AlGaAs laser diodes at 820 nm.
One of the major problems of semiconductor laser diodes of the types mentioned above is the degradation in the end section area, in particular at the front facet of the laser. This degradation is believed to be caused by uncontrolled temperature increase at the mirror facet regions, especially at high power outputs, which temperature increase in turn is probably caused by unwanted carrier recombination in these regions and heating due to free carrier injection.
Consequently, ways have been sought to prevent this carrier recombination in the laser diode""s facet regions. One attempt is described in Itaya et al U.S. Pat. No. 5,343,468. It discloses a compound semiconductor laser diode with a current blocking region formed in one facet portion of the laser structure. Though this design may be advantageous for the kind of laser diodes addressed by Itaya, namely regrown/buried double heterostructure laser diodes, it is not manufacturable for ridge waveguide laser diodes of the kind addressed by the present invention. A further problem occurs when manufacturing AlGaAs laser diodes with a two step epitaxial process. Here, the quick oxidation of Al seriously interferes with the Itaya process and thus makes it rather unsuitable for industrial application
Thus, it is the main object of the invention to devise a simple and reliable design for a high power ridge waveguide laser diode which avoids the above-mentioned end section degradation to provide a stable light output power under all operating conditions. Another object is to provide an economical manufacturing method, allowing reliable mass production of such laser diodes.
A still further object is to avoid adding to the complexity of the laser diode structure and to keep the number of additional structural components of the laser diode at a minimum.
In brief, to solve the issues addressed above, the present invention creates a ridge waveguide laser diode with so-called unpumped end sections, thus effecting a relaxation of the high stress areas in the vicinity of the laser""s facets. An appropriate manufacturing process for defining these unpumped end sections to block the induction of high current densityxe2x80x94and therefore stressesxe2x80x94into the end sections forms another aspect of the invention.
A ridge waveguide laser diode structured according to the present invention exhibits an extreme improvement over prior art laser diodes, especially with regard to its long-term stability and reliability, as will be shown.
This significant improvement is effected by just a minor increase in manufacturing complexity so that conventional manufacturing equipment may be used and usual manufacturing processes may be applied. Also, the laser diode itself has the same dimensions as before, thus avoiding any packaging changes or problems.
As briefly mentioned above, the design idea of one or two unpumped end section(s) in a ridge waveguide laser diode is to relax high stress areas close to the laser""s facets. Due to a crystal inhomogeneityxe2x80x94often produced by cleaving the laser cavityxe2x80x94and high optical power densities within these regions, especially in the front facet/mirror vicinity, the regions adjacent to the mirrors are believed to be the weakest points within the whole waveguide in terms of reliability.
Particularly in the front facet region, an increased failure rate at very high optical output powers can be observed. The same was found, though to a lesser degree, in the vicinity of the back mirror. Since the material degradation in these high stress region is accelerated by a combination of optical power density and electrical current density, it seems advantageous to reduce the latter. This is effected by establishing one or two unpumped end section(s) according to the present invention.